Satellite communications systems typically include a number of earth stations with each station transmitting signals to and receiving signals from a satellite located in a geostationary orbit. The earth station typically employs a single antenna which serves the dual functions of radiating the transmit signal and capturing the receive signal. In a reflector antenna system, an antenna feed combines the transmit and receive signals which then are coupled to an antenna horn and directed to the reflector antenna.
The amount of electromagnetic spectrum available for earth station to satellite communications is limited. Multiple users are able to use different frequency channels in an assigned frequency band. For example, earth station C-Band users transmit in the band of 5.9-6.4 GHz and receive in the band of 3.7-4.2 GHz. However, as the number of satellite users has increased, additional measures have been used to further expand the use of satellite resources.
Frequencies can be reused, for example, if linear polarization separation is used to reduce unwanted interference from co-channel or adjacent channel sources. Many satellite earth stations operate either with "co-polarization", that is, both transmit and receive signals having the same relative polarization or using "cross-polarization", that is, transmit and receive polarizations are orthogonal relative to each other. Other relative angular orientations between the transmit and receive signals are also possible.
The antenna feed of an earth station typically couples the transmit signal to the antenna and couples the receive signal from the antenna to the receiver. Desirable characteristics of an antenna feed include high isolation between transmit and receive ports and low insertion loss. In addition, any feed should be relatively simple in construction to thereby allow for economical manufacture. Compact size and weight, and ruggedness are also desirable.
Microwave devices are known in the art for combining cross-polarized transmit signals. For example, U.S. Pat. No. 2,975,380 to Scharfman discloses a broadband waveguide transducer capable of coupling orthogonal modes from two sources. In addition, devices are known which combine co-polarized signals of different frequencies. For example, U.S. Pat. No. 4,504,805 to Ekelman et al. discloses a multi-port combiner having a common circular waveguide with fixed rectangular slots coupled to side arm waveguides for transmitting and receiving microwave co-polarized signals in at least two frequency bands.
Cross-polarization feeds are known in the art as are co-polarization feeds. However, each type of feed operates at only one configuration. Thus, one antenna feed is needed for co-polarized signals and an entirely different feed is required for cross-polarized signals. Therefore, to convert between co-polarization and cross-polarization, a technician must physically substitute the antenna feeds in the field. Moreover, applicant is unaware of any feed which may operate in either polarization configuration by simple field adjustment.
In addition, filters are typically required on all ports to provide adequate isolation between the transmit and receive ports. A filter, such as a cavity filter, adds to the cost of the feed and also increase insertion loss. For example, a four-port dual polarization frequency diplexer, including various types of filters, is shown in U.S. Pat. No. 4,912,436 to Alford et al.